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Figure (8) illustrates the effect POTENTIAL OF CATHODE RELATIVE TO HEATER NEGATIVE (V k, h- VOLTS)
upon the characteristics of vary-
ing the heater voltage (of the
same BM30 as was used for ob-
taining the values for Figs. (6)
and (7)) from 3 to 5.6 volts when
the valve was otherwise operated
at V a = 6 volts, V g1 = 4 volts,
V g2 = -2 volts, V k,h- = 4 volts.
In this case we find that as the
heater voltage is raised from the
value V h = 3 volts:
(i) The anode current increases
approximately linearly. Fig. (9)
(ii) The screen current increases rapidly as V h increases Figure 9 illustrates that the control grid current of
to 3.4 volts and then rises more steadily, and each valve was at a steady minimum value when its
approximately linearly, from V h = 3.5 to 5.6 volts. cathode potential was within the range 4 to 10 volts
positive relative to heater negative (i.e. 0 to 6 volts
(iii) The control grid current is reasonably constant for positive relative to heater positive), and it is seen that
the range V h = 3 to 4.6 volts and is particularly such stability commences as soon as the cathode poten-
steady from V h = 3.6 to 4.2 volts. As tial is made to exceed the mean potential (2 volts) of
V h increases through the value 4.6 volts the control the heater. As the value of V k,h- decreases from
grid current rapidly decreases and becomes zero +2 to -10 volts it is noted that only slight variation
at V h = 4.87 volts, thereby indicating that the free of the control grid current is produced in valve A
grid potential is -2.0 volts at operating conditions whereas the control grid current of valve B is appreci-
V a = 6 volts, V g1 = 4 volts, V h = 4.87 volts, and ably altered and soon becomes outside the specified
V k,h- = 4 volts. limit. It appears that as soon as V k,h- exceeds 2 volts
the majority of the positive ions emitted by the heater
It is noted that the control grid current remains are prevented from reaching the control grid and that
constant within the range +0.2 and -0.4 volts of such control reaches its maximum when the cathode
the 4 volts specified heater voltage of the valve. potential is positive relative to all parts of the heater.

Figures (9) and (10) indicate
the results of an investigation into
the effect of the potential differ-
ence between the cathode and the
heater of the BM30 upon the
control grid current and the free
grid potential respectively when
the valve is operated at V a = 6
volts, V g1 = 4 volts, V h = 4
volts, and V g2 = -2 volts. The
curves selected for inclusion are
a pair which show the extreme
effects in so far as the character-
istics of valve A showed little
change whereas those of valve B
were appreciably altered.

Fig. (10)

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